Well, since the options are only male or female, "Other" could refer to a hermaphrodite.

A hermaphrodite. A hermaphrodite is someone with both male and female sex organs. Having an account myself, I know you can't pick both. You're answer would be "Other". Indeed, there are people like this who exist, although it's definitely not common. So don't get all "What is this I don't even...", because it happens.